Q-Chem is a state-of-the-art commercial quantum chemistry program that is used to model molecular processes over a wide range of disciplines, including biology, chemistry, and materials science. Among all the quantum models, density-functional theory (DFT) is the most-widely applied. In this SBIR application, we seek to apply the most recent advances in the molecular DFT calculation to the periodic systems. The current state-of-art programs capable of handling the PBC has major deficiencies in accuracy and efficiency, and lacks the capability for very large unit cells. In recent years, we have developed some new DFT methods for molecular systems that have improved the efficiency of the DFT calculation by several folds without loss of accuracy. In this Phase I proposal, we plan to extend those methods to the periodic systems. We will apply our multiresolution exchange-correlation (mrXC) method to the periodic systems. This method combines seamlessly the two different types of numerical grid with traditions in periodic systems and the molecular quantum chemistry, respectively. mrXC takes advantage of both grids without sacrifice of accuracy. For the Coulomb problem, we will develop a new method to treat the four-center integrals efficiently, in addition to the applying the Fourier-transform Coulomb method. The diagonalization of the Hamiltonian becomes a computational bottleneck for unit cells containing hundreds of atoms or more. We will apply our recently developed absolutely localized molecular orbitals model, which has been very effective for the study of molecular clusters. At the end of this project, Q-Chem will become a uniquely efficient and accurate electronic structure package for large periodic systems. PUBLIC HEALTH RELEVANCE: This project aims to develop and implement efficient and accurate DFT methods for periodic systems. DFT is at the core of molecular modeling and is applied widely in biological research/development and in drug discovery. Periodic systems are routinely used in biological simulations. There is a lack of efficient and accurate DFT methods for this type of systems. The efficient and accurate application of DFT to the periodic systems will significantly increase researchers'quality of work and extend the application scope of quantum simulation.